THE TANGANYIKA PROBLEM. 
355 
long ago, in fact, that the animals composing this fauna are 
now for all practical purposes so many lingering shadows of 
the past, and consequently the discovery of the identity 
which exists between the halolimnic and marine Jurassic 
shells is after all nothing but what we might have been led 
to expect from a consideration of other lines of evidence ; 
the comparison merely puts the finishing touch to a series 
of investigations, all of which persistently converge towards 
the same point and gives us a direct indication of the 
geological age during which Tanganyika ceased to be 
directly stocked with organisms from the ancient seas. 
Up to the present time this brings us to the conclusion of 
the whole matter, but it may be useful to briefly recapitulate 
the broader results of the foregoing enquiries. 
It has been seen that a vast succession of related 
geological changes have gone on throughout the equatorial 
regions of Africa, and that up to the present time these 
changes have resulted in the incipient formation of a great 
mountain chain. These same changes have gone on at 
different times, and where there were once obviously vast 
and deep depressions, the old aqueous deposits of the 
interior have been broken and thrust up into the crests of 
the Great Central African Range, now thousands of feet 
above the sea. There is no evidence of the extension of 
Tanganyika to the north or east, but there is, as we have 
seen, an obvious probability that it did extend to the west 
along the Congo depression. The geology of the districts 
west of Tanganyika and of the upper and lower portion of 
the Congo basin thus afford a most promising field for 
future exploration, and the same may be said of the whole 
Congo system, for so far as it is at present known, it has 
yielded the most intensely interesting results. 
During the course of the geological changes which have 
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